A skirt or jeans? Sneakers or flip-flops? Where, exactly, is that perilous line between looking cool and looking like your mom dressed you?

Of all the challenges for girls entering the sixth or seventh grade -- new building, new kids, new rules -- there is none so daunting as The Outfit.

These first-day ensembles are debated for days, sometimes weeks, endlessly vetted by best friends, older siblings and parents. Anxiety over picking the right look continues until the bus or carpool pulls up.

"You're always thinking about it. All summer," said Caleigh Pickett, 12, who plans to show up to the first day of seventh grade at Franklin Middle School in Chantilly on Tuesday in jeans and a white embroidered shirt from Abercrombie & Fitch. "It's the most important day. You want to make a good impression."

Starting middle school is a turning point in the lives of many girls. In addition to the thrill of having their first lockers, they quickly transition from allowing their moms to buy them "cute coordinated outfits" at department chains to doing their own shopping at a handful of stores their friends have deemed cool, said Sandra Markus, a professor at the Fashion Institute of Technology in New York.

Most of these girls have already hit one growth spurt, making back-to-school shopping almost a necessity. And in the next year or two, they will suddenly have figures, push dress code rules and struggle to find their own sense of style, she said. Boys are going through changes of their own, but fashion usually doesn't play as important a role for them.

"Fashion is not what defines a boy and doesn't define their pecking order in school," Markus said. "For girls -- I hate to say it -- it's all about what they are wearing."

And until these girls figure out their style, a discovery process that continues well into high school, they typically want to wear what everyone else is wearing. Brand names take on an outsize importance, and groups of friends often wear the same thing -- making those not in the group feel even more left out.

"In middle school, girls would organize little plans with their friends and text each other. Like, 'Everyone wear a dress tomorrow,' " said Raina Aide, 14, who attended Glasgow Middle School in Alexandria and will be a freshman at J.E.B. Stuart High School on Tuesday.

For the first day, many girls opt for something that's not too edgy but still shows some personality, signaling to others: This is who I am today. Please like me!

"When people think of Kristina Batal, I want them to think, 'Oh, she's someone you can count on. She has a great fashion sense. She's a good friend,' " said Kristina, 12, who is excited -- nervous-excited -- about starting seventh grade at Robert Frost Middle School in Fairfax on Tuesday.